Microlithography is used for producing microstructured components such as, for example, integrated circuits or LCDs. The microlithography process is carried out in what is called a projection exposure apparatus, which comprises an illumination device and a projection lens. The image of a mask (reticle) illuminated by the illumination device is in this case projected by the projection lens onto a substrate (e.g. a silicon wafer) coated with a light-sensitive layer (photoresist) and arranged in the image plane of the projection lens, in order to transfer the mask structure to the light-sensitive coating of the substrate.
In projection lenses designed for the EUV range, i.e. at wavelengths of e.g. approximately 13 nm or approximately 7 nm, owing to the lack of availability of suitable light-transmissive refractive materials, mirrors are used as optical components for the imaging process.
Among others, the operation of mirrors under grazing incidence is known. Such mirrors operated under grazing incidence, which it is desirable to use chiefly in respect of the comparatively high obtainable reflectivities (e.g. of 80% and more), are understood here and in the following to mean mirrors for which the reflection angles, which occur during the reflection of the EUV radiation and relate to the respective surface normal, are at least 65°. Sometimes, such mirrors are also referred to in an abbreviated fashion as GI mirrors (“grazing incidence”).
A problem arising in practice in the operation of a microlithographic projection exposure apparatus is, among other things, that undesired local variations of the intensity in the field plane and/or pupil plane occur which result in optical aberrations and therefore in a deterioration of the efficiency of the projection exposure apparatus. One of the causes of this undesired intensity variation lies in the variations of the reflectivity across the respective mirror, which variations are comparatively strong in particular in the aforementioned mirrors operated under grazing incidence and in turn are caused by significant variations of the angle of incidence across the optically effective surface of the respective mirror.
To overcome the problem described above, various approaches are known in practice by which the said intensity variations are compensated. Examples of these approaches are the deliberate detuning of the reflection layer system in the mirrors operated with substantially perpendicular incidence, or the deliberate provision, by other means, of a lateral transmission variation, e.g. by use of additional layers.
However, disadvantages arising in practice in such approaches generally include loss of light and an increased sensitivity to manufacturing fluctuation, and also a greater complexity of the process of producing the relevant mirrors used for correction. Moreover, in the case where the described intensity variation is caused by a GI mirror, the profile of the reflectivity of the respective GI mirror, which is dependent on the angle of incidence, is itself subject to manufacturing fluctuations, which in turn necessitate repeated adaptation of the respective correction in each individual case.
With regard to the prior art, reference is simply made to EP 1 282 011 B1, U.S. Pat. No. 6,333,961 B1, U.S. Pat. No. 6,833,223 B2, WO 2015/135726 A1, WO 2012/113591 A1, U.S. Pat. No. 8,279,404 B2, U.S. Pat. No. 8,605,257 B2, U.S. Pat. No. 8,587,767 B2, EP 2 100 190 B1, U.S. Pat. No. 8,928,972 B2 and US 2013/0038929 A1.